This invention relates to SnO.sub.2 conductive elements which are particularly useful in thin film photovoltaic cells.
Effective conversion from expensive petroleum-based energy sources to solar energy sources, such as photovoltaic cells, has been delayed by two factors--the cost of mass-producing such and the low conversion efficiency achieved by such cells. Any improvement in either factor is capable of moving industry towards the use of more solar cells, and an improvement in both has been a long-sought goal.
The use of p-n cadmium telluride--cadmium sulfide photovoltaic cells having thin layers as described in U.S. Pat. No. 4,207,119 has considerably improved the efficiency of the cells. A limiting factor to further improvements in the efficiency of such cells is the window electrode through which the solar cell is illuminated.
Conventionally, the window electrode comprises a glass support with a transparent and conductive coating as for example In.sub.2 O.sub.3 such as is available under the trademark Nesatron.RTM. from PPG Industries. These materials and others such as Cd.sub.2 SnO.sub.4 and CdSnO.sub.3 yield films of low resistivity and high transmittance, but such materials are not readily available and the processes required for their preparation make them extremely expensive for use in photovoltaic cells.
Glass electrodes containing more available and less expensive coatings have been sought. R. G. Livesey, E. Lyford and H. Moore, J. of Physics E: J. of Scientific Instruments, 1, 947 (1968) describe a transparent conductive tin oxide film on glass prepared by flowing oxygen through a flask of heated SnCl.sub.2.2H.sub.2 O onto glass substrates. These tin oxide films had 85% transmittance but electrical resistivities of 100-500 ohm per square. Films having lower resistance were acknowledged by the authors to be undesirable due to haze.
James Kane, H. P. Schwizer and Werner Kern in Volume 123, No. 2 of J. Electrochem. Soc: Solid-State Science and Technology, pages 270-276 (February, 1976) describe the use of a soda-lime glass support for a tin oxide film wherein the soda-lime glass surface is necessarily treated to remove sodium from the soda-lime glass at the surface to prevent haze from forming.
U.S. Pat. No. 3,880,633 describes a tin oxide film on glass prepared by spraying a solution of SnCl.sub.2 in methanol with small amounts of ammonia bifluoride. This method discloses an acid pretreatment of the glass support prior to the application of the SnO.sub.2 layer, forming a silica film over the support, to not only lessen the resulting haze in the SnO.sub.2 layer and glass support but also to be instrumental in obtaining a satisfactory layer resistance and high transmittance. This method achieves 78% transmittance and electrical resistance as low as 10 ohm/square, but the resulting tin oxide films are still hazy. The appearance of haze on the electrode film causes light scattering (a loss of transmittance).
Thus, the prior art is replete with references to the desirability of using soda-lime glass as the support for conductive elements as this material is extremely inexpensive, but has not found an acceptable way of using the material without involving the expense of first pretreating the support to remove sodium so that the support is no longer soda-lime glass per se or to add a layer of silica on the soda-lime glass. A substantially haze-free element formed from a support of soda-lime glass containing a layer of SnO.sub.2 directly on the soda-lime glass support is deemed to be highly useful in this art.